Solid-state image pickup device

ABSTRACT

A solid-state image pickup device  1  includes a semiconductor substrate  10 , light receiving unit  14  and light shielding film  20 . The solid-state image pickup device  1  is back surface incident type and photoelectrically converts light indent on the back surface S 2  of the semiconductor substrate  10  from an object into electrical charges and receives electrical charges produced by photoelectric conversion at the light receiving unit  14  to image the object. The light receiving unit  14  forms a PN junction diode with the semiconductor substrate  10 . The light shielding film  20  is provided over a front surface S 1  of the semiconductor substrate  10  so as to cover the light receiving unit  14 . The light shielding film  20  serves to shield light incident on the front surface S 1  from the outside of the solid-state image pickup device  1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state image pickup device.

2. Description of the Related Art

Japanese Patent Laid-Open No. 2002-33469 describes a back surfaceincident type of solid-state image pickup device. In the solid-stateimage pickup device, light incident on the back surface of asemiconductor substrate from an object is photoelectrically convertedinside the semiconductor substrate. Electric charges produced by theconversion are received by a light receiving unit to image the object.

Japanese Patent Laid-Open No. 2000-252452 and Japanese Patent Laid-OpenNo. 03-148172 in addition to Japanese Patent Laid-Open No. 2002-33469are known as a prior art document related to the present invention.

The present inventor has found that the above solid-state image pickupdevice had the following problems to be solved. That is to say, in theabove solid-state image pickup device, stray light may enter asemiconductor substrate on which a light receiving unit is provided fromthe face (surface) on the side opposite to the back surface being animaging surface. Stray light entering the semiconductor substrategenerates a false signal, degrading a pickup image.

SUMMARY

According to one aspect of the present invention, there is provided asolid-state image pickup device photoelectrically converting lightincident on the back surface of a semiconductor substrate into signalcharges to image an object, the solid-state image pickup devicecomprising: a light receiving unit provided on side of a front surfaceof the semiconductor substrate in the semiconductor substrate, forming aPN junction diode with the semiconductor substrate and receiving thesignal charges produced by the photoelectric conversion; and a lightshielding film provided over the front surface of the semiconductorsubstrate so as to cover the light receiving unit, the light receivingunit being provided to prevent light incident on the front surface fromthe outside of the solid-state image pickup device.

According to another aspect of the present invention, there is provideda solid-state image pickup device comprising: a semiconductor bodyhaving a first main surface on which light is incident, thesemiconductor body further having a second main surface opposite to thefirst main surface, a diffusion region formed at the second main surfaceof the semiconductor body to produce a electrical signal response to thelight, and a light shielding film provided over the diffusion region.

A light shielding film is provided over the front surface of asemiconductor substrate in the solid-state image pickup device. Thisprevents stray light from entering the semiconductor substrate from thesurface.

The present invention realizes the solid-state image pickup devicecapable of providing excellent pickup images.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and features of the presentinvention will be more apparent from the following description ofcertain preferred embodiments taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a cross section illustrating a first embodiment of asolid-state image pickup device according to the present invention;

FIG. 2 is a cross section illustrating a second embodiment of asolid-state image pickup device according to the present invention; and

FIG. 3 is a cross section illustrating a third embodiment of asolid-state image pickup device according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of a solid-state image pickup device according tothe present invention are described in detail below with reference tothe drawings. In the description of the drawings the same constituentelements are given the same reference numerals to omit duplicateddescription.

First Embodiment

FIG. 1 is a cross section illustrating a first embodiment of asolid-state image pickup device according to the present invention. Asolid-state image pickup device 1 includes a semiconductor substrate(semiconductor body) 10, light receiving unit (diffusion region) 14 andlight shielding film 20. The solid-state image pickup device 1 is backsurface incident type. More specifically, the solid-state image pickupdevice 1 at inside of the semiconductor substrate 10 photoelectricallyconverts light indent on the back surface (first main surface) S2 of thesemiconductor substrate 10 from an object into electrical charges(signal charges) and receives electrical charges produced byphotoelectric conversion at the light receiving unit 14 to image anobject. Image pickup is preferably performed with the object directlybrought into contact with the above back surface S2. The object is, forexample, a finger.

The semiconductor substrate 10 in the present embodiment is a p-typesilicon substrate. The semiconductor substrate 10 includes a p-type wellregion 12 in which the light receiving unit 14 is formed. The lightreceiving unit 14 is an n-type impurity diffusion layer (a firstimpurity diffusion layer) and forms a PN junction diode with thesemiconductor substrate 10.

The light shielding film 20 is provided over the front surface (secondmain surface) S1 of the semiconductor substrate 10 so as to cover thelight receiving unit 14. Here, the front surface S1 is defined as anelement formation face of the semiconductor substrate 10, and the backsurface S2 is defined as the surface opposite to the front surface S1.The light shielding film 20 covers the whole area where the lightreceiving unit 14 is provided. In the present embodiment, the lightshielding film 20 covers, in particular, almost the whole area of thesurface S1 of the semiconductor substrate 10. The light shielding film20 serves to shield light incident on the surface S1 from the outside ofthe solid-state image pickup device 1.

On the semiconductor substrate 10 is provided an interconnect layer 30,in which the aforementioned light shielding film 20 is provided. Theinterconnect layer 30 includes interconnects 32, 34 and 36. The lightshielding film 20 is formed by the same material as the interconnects32, 34 and 36. The materials to be used in the light shielding film 20include Al, Cu, W or Ti, for example.

A gate insulating film 42 is formed on the front surface S1 of thesemiconductor substrate 10. The gate insulating film 42 is providedadjacent to the foregoing light receiving unit 14. A gate electrode 44connected to the interconnect 34 is formed on the gate insulating film42. The gate insulating film 42 and gate electrode 44 are formed by, forinstance, silicon oxide and poly-silicon respectively.

Furthermore, an n-type impurity diffusion layer 46 (a second impuritydiffusion layer) is provided beyond the gate insulating film 42 and thegate electrode 44 on the side opposite to the light receiving unit 14.In other words, the gate insulating film 42 and the gate electrode 44are interposed between the n-type impurity diffusion layer 46 and thelight receiving unit 14. The n-type impurity diffusion layer 46 isconnected to the interconnect 36. The gate insulating film 42, gateelectrode 44 and n-type impurity diffusion layer 46 form a field effecttransistor (FET) with the light receiving unit 14. In the FET, the gateelectrode 44 and n-type impurity diffusion layer 46 function as a resetgate and reset drain respectively. In addition, the FET is isolated fromother elements by an element isolation region 62. The element isolationregion 62 is, for example, a local oxidation of silicon (LOCOS) regionor shallow trench isolation (STI) region.

A source follower amplifier 50 is connected to the light receiving unit14 via an interconnect 64. Although the interconnect 64 is also formedin the interconnect layer 30, the interconnect 64 and source followeramplifier 50 are illustrated using the circuit diagram in FIG. 1.

The source follower amplifier 50 includes an FET 52 for a selectiveswitch, FET 54 for detection and FET 56 for a load. These FETs areprovided between a high-potential power supply terminal (Vdd) and alow-potential power supply terminal (Vss). Specifically, the FET 54 fordetection, FET 52 for a selective switch and FET 56 for a load areconnected to each other in series in this order from the power supplyterminal (Vdd) to the power supply terminal (Vss). An output terminal 58of the source follower amplifier 50 is connected to the drain terminalof the FET 56 for a load (or, to the terminal on the side of the FET 52for a selective switch).

The operation of the solid-state image pickup device 1 is thendescribed. Firstly, applying a high pulse to the gate electrode 44 isperformed to reset the electric potential of the light receiving unit14. Secondly, applying a low pulse to the gate electrode 44 is performedto stores electric charges produced by photoelectric conversion in thelight receiving unit 14. Then, the electric potential of the lightreceiving unit 14 reaches a value depending on the quantity of storedelectric charges, and an electric potential in response to the electricpotential of the light receiving unit 14 is output from the outputterminal 58 of the source follower amplifier 50.

The effects obtained by the solid-state image pickup device 1 aredescribed. In the solid-state image pickup device 1, the light shieldingfilm 20 is provided over the front surface S1 of the semiconductorsubstrate 1. This prevents stray light from entering the semiconductorsubstrate 10 from the surface S1, which realizes the solid-state imagepickup device 1 capable of providing an excellent pickup image.

Covering the whole area where the light receiving unit 14 is provided bythe light shielding film 20 enables pickup images to be effectivelyprevented from deteriorating due to stray light in particular.Furthermore, in the present embodiment, covering almost the whole areaof the front surface S1 of the semiconductor substrate 10 by the lightshielding film 20 permits stray light to be surely prevented fromentering the semiconductor substrate 10.

The light shielding film 20 is provided in the interconnect layer 30 andformed of the same material as the interconnects 32, 34 and 36. Thisallows the light shielding film 20 to be formed in an interconnectforming process to facilitate forming the light shielding film 20, orproducing the solid-state image pickup device 1. For instance, the lightshielding film 20 is enabled to be formed simultaneously with anelectrode pad (not shown).

The light receiving unit 14, gate insulating film 42, gate electrode 44and n-type impurity diffusion layer 46 form an FET. Therefore, aconfiguration for resetting the electric potential of the lightreceiving unit 14 can be realized by a simple configuration.

Japanese Patent Laid-Open No. 03-148172 (hereafter, referred as thedocument) discloses a solid-state image pickup device in which a lightshielding film is provided on the side of the surface of a semiconductorsubstrate. The solid-state image pickup device in the document, however,is different from the solid-state image pickup device 1 using the PNjunction diode as a light receiving unit in that it uses a Schottkybarrier diode as a light receiving unit. The solid-state image pickupdevice described in the document limits light to be used to infraredlight (with a wavelength of approximately 5 μm or shorter), so that itcannot use the PN junction diode. In other words, if the solid-stateimage pickup device in the document uses the PN junction diode, a bandgap is increased, so that the light receiving unit cannot sufficientlyreceive infrared light.

Since the solid-state image pickup device disclosed in the document isdirected to detecting infrared light as stated above, it is sensitive toheat ray. If an object to be imaged is a finger, the solid-state imagepickup device is influenced by heat ray from the whole finger. For thatreason, the solid-state image pickup device is not suited for imaging afingerprint. In this respect, the solid-state image pickup device 1 issuitably applied to imaging a fingerprint.

Since the solid-state image pickup device in the document is chargecoupled device (CCD) type, its interconnect layer includes single layerof interconnect. Because of this, space where interconnect can bedisposed in the interconnect layer is limited, which makes it difficultto form a light shielding film in the interconnect layer. Actually, thelight shielding film is provided on the upper portion of theinterconnect layer in the solid-state image pickup device in thedocument. Consequently, as is not the case with the solid-state imagepickup device 1, the light shielding film cannot be formed in theinterconnect forming process, increasing the number of processes forforming the light shielding film.

Second Embodiment

FIG. 2 is a cross section illustrating a second embodiment of asolid-state image pickup device according to the present invention. In asolid-state image pickup device 2, a light shielding film 20 and aninterconnect 34 are integrally formed. In other words, the lightshielding film 20 is part of an interconnect in an interconnect layer 30and functions both as a light shielding film and as an interconnect. Inthe present embodiment, the light shielding film 20 covers the wholearea where the light receiving unit 14 is provided. However, the lightshielding film 20 does not cover almost the whole area of the frontsurface S1 of the semiconductor substrate 10 as shown in FIG. 2. Otherconfigurations and operations of the solid-state image pickup device 2are the same as those of the solid-state image pickup device 1.

According to the solid-state image pickup device 2 thus formed, thelight shielding film 20 and interconnect 34 are formed together toreduce the number of manufacturing processes. Other effects of thesolid-state image pickup device 2 are the same as those of thesolid-state image pickup device 1. It is to be understood that the lightshielding film 20 may be integrated with interconnects (for example,interconnects 32 and 36) except the interconnect 34 in the interconnectlayer 30 in the present embodiment.

Third Embodiment

FIG. 3 is a cross section illustrating a third embodiment of asolid-state image pickup device according to the present invention. In asolid-state image pickup device 3, an n-type impurity diffusion layer 48(a second impurity diffusion layer) is formed in the semiconductorsubstrate 10 on the side of a front surface S1 of the semiconductorsubstrate 10. The n-type impurity diffusion layer 48 is providedadjacent to a light receiving unit 14. Furthermore, a gate insulatingfilm 42 is provided adjacent to the n-type impurity diffusion layer 48on the front surface S1 of the semiconductor substrate 10. A gateelectrode 44 is formed on the gate insulating film 42. Stillfurthermore, an n-type impurity diffusion layer 46 (a third impuritydiffusion layer) is provided beyond the gate insulating film 42 and thegate electrode 44 on the side opposite to the n-type impurity diffusionlayer 48. In other words, the gate insulating film 42 and the gateelectrode 44 are interposed between the n-type impurity diffusion layer46 and n-type impurity diffusion layer 48.

The n-type impurity diffusion layer 48, gate insulating film 42, gateelectrode 44 and n-type impurity diffusion layer 46 form an FET. Also inthe FET, the gate electrode 44 and n-type impurity diffusion layer 46function as a reset gate and reset drain respectively. Otherconfigurations and operations of the solid-state image pickup device 3are the same as those of the solid-state image pickup device 1. Theeffects of the solid-state image pickup device 3 are also the same asthose of the solid-state image pickup device 1. Also in the presentembodiment, as is the case with the solid-state image pickup device 2,the light shielding film 20 may be integrated with the interconnects inthe interconnect layer 30.

The solid-state image pickup device according to the present inventionis not limited to the above embodiments, but may be modified in variousforms. Although the p-type semiconductor substrate and n-type lightreceiving unit are taken as examples in the above embodiments, then-type semiconductor substrate and p-type light receiving unit may beused.

It is apparent that the present invention is not limited to the aboveembodiments, but may be modified and changed without departing from thescope and spirit of the invention.

1. A solid-state image pickup device photoelectrically converting lightincident on a back surface of a semiconductor substrate into signalcharges to image an object, the solid-state image pickup devicecomprising: a light receiving unit provided on side of a front surfaceof said semiconductor substrate in said semiconductor substrate, saidlight receiving unit forming a PN junction diode with said semiconductorsubstrate and receiving said signal charges produced by thephotoelectric conversion, said front surface being opposite to said backsurface, said semiconductor substrate being a first conductivity type,and a region between said light receiving unit and said back surface isfree of a second conductivity type; a light shielding film provided oversaid front surface of the semiconductor substrate so as to cover thelight receiving unit, said light shielding film being provided toprevent light incident on said front surface from the outside of thesolid-state image pickup device; and a MOS transistor formed adjacentlyto said light receiving unit, wherein said light shielding film coverswhole area where said light receiving unit is provided.
 2. Thesolid-state image pickup device according to claim 1, wherein said lightshielding film covers substantially whole area of said front surface ofthe semiconductor substrate.
 3. The solid-state image pickup deviceaccording to claim 1, further comprising an interconnect layer providedon said front surface of the semiconductor substrate and includinginterconnects, wherein said light shielding film is formed of the samematerial as said interconnects.
 4. The solid-state image pickup deviceaccording to claim 3, wherein said light shielding film is provided insaid interconnect layer.
 5. The solid-state image pickup deviceaccording to claim 5, wherein said light shielding film is part of saidinterconnects.
 6. The solid-state image pickup device according to claim1, wherein said back surface of said semiconductor substrate is asurface with which said object is directly brought into contact.
 7. Thesolid-state image pickup device according to claim 1, wherein saidobject is a finger.
 8. The solid-state image pickup device according toclaim 1, wherein said light receiving unit is a first impurity diffusionlayer.
 9. The solid-state image pickup device according to claim 8,further comprising: a gate insulating film provided adjacent to saidlight receiving unit on said front surface of said semiconductorsubstrate; a gate electrode provided on said gate insulating film; and asecond impurity diffusion layer provided on side of said front surfaceof said semiconductor substrate in said semiconductor substrate, saidsecond impurity diffusion layer being disposed so that said gateinsulating film and said gate electrode are interposed between saidsecond impurity diffusion layer and said light receiving unit; whereinsaid light receiving unit, said gate insulating film, said gateelectrode and said second impurity diffusion layer form a field effecttransistor.
 10. The solid-state image pickup device according to claim8, further comprising: a second impurity diffusion layer providedadjacent to said light receiving unit on side of said front surface ofsaid semiconductor substrate in said semiconductor substrate; a gateinsulating film provided adjacent to said second impurity diffusionlayer on said front surface of said semiconductor substrate; a gateelectrode provided on said gate insulating film; and a third impuritydiffusion layer provided on side of said front surface of saidsemiconductor substrate in said semiconductor substrate, said thirdimpurity diffusion layer being disposed so that said gate insulatingfilm and said gate electrode are interposed between said second impuritydiffusion layer and said third impurity diffusion layer; wherein saidsecond impurity diffusion layer, said gate insulating film, said gateelectrode and said third impurity diffusion layer form a field effecttransistor.
 11. A solid-state image pickup device comprising: asemiconductor body having a first main surface on which light isincident, said semiconductor body further having a second main surfaceopposite to said first main surface; a diffusion region formed at saidsecond main surface of said semiconductor body to produce a electricalsignal response to the light, said diffusion region having a secondconductivity type; a light shielding film provided over said second mainsurface of said semiconductor body to cover said diffusion region; and aMOS transistor formed adjacently to said diffusion region, wherein saidsemiconductor body having a first conductivity type, and a regionbetween said light receiving unit and said back surface is free of saidsecond conductivity type, wherein said light shielding film covers wholearea where said light receiving unit is provided.
 12. The solid-stateimage pickup device according to claim 11, wherein said light shieldingfilm is formed over a substantial whole of said second main surface.